Chess Board Model

The Idea

Cut the question in two. A piece answers only about geometry — is this offset shaped like my move? The board answers about occupancy — is the path clear, is the target one of mine, is it my turn?

Neither object needs the other's data, so a new piece is a new class and nothing else.

Real-World Example

A choreographer and a stage manager. The choreographer says a dancer may travel diagonally; the stage manager says that wing is blocked tonight. A step happens only when both agree.

The Code

class Rook:
    def shape_ok(self, a, b): return a[0] == b[0] or a[1] == b[1]

class Knight:
    def shape_ok(self, a, b):
        return sorted((abs(a[0] - b[0]), abs(a[1] - b[1]))) == [1, 2]

class Board:
    def __init__(self, pieces): self.pieces = pieces      # square -> piece
    def legal(self, a, b):                                # geometry, then state
        piece = self.pieces.get(a)
        return bool(piece) and piece.shape_ok(a, b) and b not in self.pieces

b = Board({(0, 0): Rook(), (1, 0): Knight()})
print(b.legal((0, 0), (0, 5)), b.legal((1, 0), (2, 2)), b.legal((0, 0), (1, 0)))
# True True False

The Tradeoff

Per-piece geometry keeps the board tiny, but every sliding piece repeats the walk-the-squares-between loop, and castling, en passant and check span pieces — so they land on the board anyway. A move-rule table is more uniform and much harder to read.